1. Field of the Invention
The present invention relates generally to measuring polymer film thickness and more specifically to using coherent monochromatic light to measure polymer film thickness during the manufacturing process.
2. Background of the Invention
Polymer films are used in various product applications requiring different characteristics. These films are formed into films, or webs, of various widths and thicknesses depending upon the use to which they will be put. Some considerations in the manufacture are tensile strength, durability, use as a substrate for other films, ability to be colorized or accept pigments, ability to etched, clarity, and ease of manufacture.
The intended use of the film will generally control the needed thickness of the film. A number of measuring systems including mechanical stylus calipers, beta radiation gauges, filtered broad band light source calipers, x-ray radiation gauges, and gama radiation gauges are available to accomplish these measurements.
Mechanical stylus calipers, although simple in nature, are impractical for use during manufacturing processes. This method uses a sample taken from polymer film stock rolls, measures the thickness of that sample, and relates it back to the stock roll in general. Although accurate, it is a slow process and subject to error secondary to averaging the measurements over the whole stock roll.
Beta radiation gauges have been used to control manufacturing processes but have been found to be exceedingly slow and are fraught with the danger of hazardous emissions of radiation. Beta gauges have been shown to be inaccurate and are limited to the number of electrons that can be fired through the polymer film web. Inaccuracy of beta radiation gauges stems from the assumption that the material is of constant density when converting the basis weight of the polymer film to its caliper.
Filtered broadband light sources are exceedingly inefficient, using light sources that must be filtered to obtain a narrower band width that is useable in measuring polymer film thickness. A considerable amount of energy is wasted in the filtering process which is lost as heat. Despite the narrowing of the band width through the filtering process, the wavelength range is still significantly broad enough to introduce significant error within the measuring system.
X-ray and gamma radiation gauges have been developed but are impractical to use due to the exceedingly hazardous nature inherent in handling these radiation gauges.
Due to the inherent slowness and inaccuracy of the above systems, considerable waste also occurs. In the case of beta gauges that are used on-line, the rate of change is so slow that a considerable amount of material can pass through the process before a change in the manufacturing process can be effected. For the other gauges used off-line, representative samples must be obtained. The stock rolls of polymer film often contain thousands of feet of film. Since only representative samples of each roll are measured, if a sample is bad, then the entire stock roll may be considered to be bad and thrown out even if only a small segment is actually bad. Of course, the converse may also be true where the representative sample is considered good but was taken between lengths of polymer film that are actually bad.
There currently does not exist an accurate method for on-line monitoring and control of the manufacturing process to minimize, if not eliminate, poorly manufactured polymer films or considerable waste in manufacturing adequate polymer films.